1. Field of the Invention
The present invention relates to a metal plating apparatus and a process for performing such plating. More particularly, it relates to a metal plating apparatus capable of uniformly carrying out plating, even though the distribution of metal portions on an object to be plated is not uniform.
2. Background
In a conventional metal plating method, a plating metal plate serving as an anode and an object to be plated serving as a cathode are immersed in a solution containing metal ions and a predetermined voltage is applied therebetween from an external power supply. A metal is deposited on the surface of the object serving as a cathode. On the other hand, an elution of metal ions occur on the metal plate serving as an anode. For instance, in a plating apparatus 100 shown in FIG. 7, an object 102 acting as a cathode and a copper plate 104 acting as an anode are disposed and then are immersed in a plating solution tank 106 including a copper-sulfate solution. The anode of a plating power supply 108 is connected to the copper plate 104 via a switch (not shown) and an ammeter, and the object 102 on which a plating layer should be formed is connected to the cathode of the plating power supply 108. The object 102 has electrically conductive regions that are to be plated on its surface. When a voltage is applied from the plating power supply 108 between the copper plate 104 and the electrically conductive regions of the object 102, copper cations in the electrolytic plating solution flow toward the object 102 to deposit metal copper on the surface of the object 102, so that a plating layer is formed on the electrically conductive regions of the object 102.
The amount of metal deposited on the object, that is, the plating thickness distribution is determined depending on the distribution of plating-current density. Lines of electric force 110, which indicate the paths of current flowing in electrolytic plating solution, extend toward the electrically conductive region of the object 102 as shown in FIG. 8. A problem arises in that the plating thickness is not uniform because of thicker metal deposition occurring at the ends of the electrically conductive region of the object 102, which is caused by the tendency of the lines of electric force to concentrate at the ends of the electrically conductive region.
Based on recent trends wherein electronic devices are decreasing in size, increasing in packaging density, and increasing in speed, plating processes for printed circuit boards that achieve thinner conductive lines, smaller diameter via holes, smaller diameter of lands as well as multi-layers are required. In multi-layer printed circuit boards, the plating thickness on the inner surfaces of through-holes, and via holes and the like provided for interlayer connection is required to be uniform. However, in pattern areas where a plurality of via holes are concentrated, the surface area to be plated increases because the inner surfaces of the holes need to be plated as well. In ordinary plating processes of printed circuit boards, a panel, whose wiring density of the wiring pattern is not uniform, is connected as a cathode. Accordingly, a non-uniform plating thickness occurs since the current density is low in a densely patterned area to be plated resulting in a locally thin plating layer while the current density is high in a less densely patterned area resulting in a locally thick plating layer.
Various kinds of techniques for solving the above problem and obtaining a uniform plating thickness have been developed. For instance, a current concentrated on end parts of an electrically conductive region may be controlled by mounting a shielding plate, or a pattern called a xe2x80x9cdummy patternxe2x80x9d, which is not required to function as an electric circuit, may be arranged on or near the printed circuit board. However, a shielding plate can avoid only a certain extent of the concentration of the lines of electric force on the end parts, so that the plating thickness still varies depending on whether the pattern is dense or not. Further, the dummy pattern reduces production efficiency because the dummy pattern must be added to the circuit pattern but the dummy pattern does not function as a electrically productive part of the circuit.
A first aspect of the present invention is to provide a metal plating apparatus and process capable of depositing metal on an object in a uniform thickness.
A second aspect of the present invention is to provide a metal plating apparatus and process capable of plating in a uniform thickness, even if the distribution of metal portions on an object to be plated is not uniform.
A third aspect of the present invention is to provide a metal plating apparatus and process capable of performing metal plating in a uniform thickness in a short time at high electric current.
The metal plating apparatus according to the present invention includes a pair of electrically connected conductive perforated plates. The metal plating apparatus of the present invention having the above structure can substantially align in parallel the lines of electric force created in a plating bath.
Another embodiment of the metal plating apparatus according to the present invention includes an insulating adjustment plate with a plurality of windows. The metal plating apparatus of the present invention having such a structure is capable of making the current density at the metal portions to be plated constant and thus plating each of the metal portions with a uniform thickness, even if the distribution of the metal portions to be plated is not uniform.
Still another embodiment of the metal plating apparatus according to the present invention includes a pair of conductive perforated plates electrically connected to each other and an adjustment plate with a plurality of windows. The metal plating apparatus of the present invention having the above structure is capable of making the lines of electric force between the plating metal and the object uniform and parallel, as well as making the current density at each portion to be plated constant and the plating thickness uniform.
As described above, according to the metal plating apparatus of the present invention, non-uniformity of lines of electric force is improved by conductive perforated plates. Furthermore, uniform plating thickness can be obtained by providing the current density corresponding to the area to be plated using an insulating adjustment plate. The present invention can dramatically improve productivity by uniformly obtaining a desired plating thickness under the plating conditions of high current density and short processing time, which are difficult in the prior art. Moreover, conductive perforated plates and an insulating adjustment plate may be added to the conventional metal plating apparatus, so that excellent uniformity in plating thickness can be obtained.
In the metal plating apparatus of the present invention, productivity can be improved by dramatically decreasing plating thickness non-uniformity. As a result, the metal plating apparatus is applicable to plating techniques required to provide a precise and uniform plating thickness.